Welding technology for welding overlapping steel plates is finding wide use primarily in the automobile industry. Steel plates used in the automobile are plated with zinc layers for corrosion resistance. Welding apparatuses for welding such zinc-plated steel plates include gas-shielded arc welding and plasma welding apparatuses.
In such a gas-shielded arc welding and plasma welding apparatus, an inert gas is ejected from around an electrode toward a molten pool on a base material, so that the base material is welded in a given gas atmosphere, or an arc column is constricted to obtain a high temperature.
When overlapping zinc-plated steel plates are welded, plated-zinc surface layers sandwiched between the steel plates are evaporated into a zinc gas because the plated zinc layers have a lower melting temperature than the steel plates. The zinc gas is highly pressurized as there is no space it can escape into. When the steel plates are then heated, they are melted, and the zinc gas under high pressure is released into the atmosphere, forcing aside the molten steel. Therefore, the welded joint contains blowholes as marks indicative of the release of the zinc gas, resulting in a reduction in the welding quality.
To avoid the above drawback, it has been customary in the welding of overlapping zinc-plated steel plates to space the zinc-plated steel plates from each other with a clearance therebetween, and a zinc vapor generated from plated-zinc surface layers sandwiched between the steel plates is allowed to escape through the clearance. It is necessary for such a clearance to be as small as possible from the standpoint of a good welding process. The clearance is created by placing a sheet between the steel plates or applying a coating material to the steel plates.
Controlling the clearance between the steel plates in an actual production site has required an extra procedure. Therefore, there has been a demand for a method of welding zinc-plated steel plates while they are being held in intimate contact with each other without any clearance therebetween.